Motor Omnibuses and Chars.a-bancs.
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Some Points in their Structure and Mechanism.
A consideration of some of the principal details embodied in the design and construction of public-service, passenger vehicles, for town or for country use, will not be out of place at the present time, when the battle between the various systems of propulsion and transmission is daily becoming more pronounced. It might almost be said of this country, that 'there are only three distinct classes of motor omnibuses in use. These are :—Steam, petrol, and petrol-electric. A few of the characteristics of each of these systems are now given in the order named.
Steam-driven Systems.
Although the petrol-engined bus chassis is in the majority, there is no denying the fact that steam, as a motive power for passenger vehicles, has many advantages over its rival internal combustion. In a steam-propelled vehicle there is no necessity for any change-speed gear-box, with all its sliding, or epicyclie gear wheels, which, apart from their being liable to become extremely noisy, are costly to maintain. All speed regulation may be effected by simply opening or closing a throttle-valve, or by varying the point of cut-off of the steam to the cylinders. The necessary machinery for the generation and transmission of power to the road wheels consists of a generator or steam raiser, water pumps, engine, differential gear, and some means of transmitting the final drive to the road wheels. The earliest motorbuses on record were all propelled by steam engines, and, although many and great advances have been made in the design of steam engines and boilers, only a few buses employing this form of power are to be found in public-passenger service. The boiler, and the existence of steam leakages at the valves, have always been the weak points in steam vehicles, whilst the quick raising of steam, the maintenance of a working pressure safely, the keeping of a proper level of water, and the uniform retention of a clear and smokeless fire, are some of the points to which the designer has had to give considerable attention. Early constructors were, and, indeed, some of the present generation are, too apt to consider the engine as the all-important unit in the constitution of the steam bus, whereas, no matter how efficient the engine may be, if the boiler is not capable of producing and maintaining a regular supply of steam, sufficient to meet the requirements of the engine, the high efficiency of the latter is brought down to the level of the boiler. figh-pressure, superheated steam must be used for vehicles of the type now under discussion, if satisfactory results are to be obtained. Many engineers who have tried highly superheated steam, have condemned its use because, having tried it in conjunction with an engine which had been designed to use wet steam, they experienced scored valve faces and difficulty of lubrication. The mushroom-type of valve is the only one which can claim any amount of success in this direction, and, if its use had been more general, we have no hesitation in expressing the opinion that more steam-driven buses would be seen on the roads at the present time, especially if used with singleacting engines. Had more of our motor engineers given attention to the use of highly-superheated steam, it is more than probable that a more satisfactory burner would have been evolved, but work in this direction appears to have been confined entirely to •the late M. Leon Serpollet, of Paris, and to Mr. Thomas Clarkson, of Chelmsford, both of whom have spent a considerable amount of time in research. M. Serpollet is no more, though the results of his work remain for development by the Darracq-Serpollet Company, and Mr. Clarkson has not by any means finished his labours to prove that steam can give good results, as his vehicles did for many years at Torquay.
Petrol-driven Vehicles.
The care and -attention that has been bestowed on the perfection of the design of the internal-combustion engine has produced many highly successful models. Not content with the efficiency at present obtained on such engines, the Institution of Automobile Engineers has lately given serious attention to the possibilities of improvement, and, with this object in view, most informative papers have recently been read by Mr. F. W. Lanchester and Professor Callender, at the Institution's meetings. When one considers the numerous inter-dependent units which constitute the complete petrol engine, it is really surprising that such good results have been obtained so rapidly and so generally.
The type of engine, which, in the opinion of many, is most suitable for bus or char-A-banes work, is one which
runs at a comparatively slow speed : such an engine works with little vibration, and, more important still, it is much more flexible than the small-bore, high-speed engines which were the regular thing during the early days of the industry. The " pocketless " engine, with overhead camshaft, has come to the front very much of late years, owing to its higher thermal efficiency : such an arrangement involves more difficulty in lubricating the camshaft, and, if the latter is driven by means of skew gearing, there is likely to be excessive wear and consequent back-lash between the teeth, the result being noisy running.
The possibility of a petrol famine has set many an engineer on the weary quest after the paraffin carburetter, the solution of the design for which would be of great commercial service, especially where large numbers of public-service
vehicles are used. The introduction of the high-tension magneto has given us another and probably a much more reliable means of igniting the charge, than the use of accumulators, commutator, coil and high-tension distributor ever offered. The numerous wire leads, switches and '' odds and ends " of fittings necessary with accumulator ignition are rendered unnecessary in the case of the high-tension magneto. After examining the beautiful working of the modern petrol engine as it is found in any good make of bus chassis, it is almost impossible to realise that., on the same chassis, there is, probably, an antiquated changespeed gear-box with gears that slide in and out of mesh. Although " it works," could anything be more unmechanical? There is a large field for improvement in this direction, and, notwithstanding the many clever devices whose end and object is abolition of the sliding gear, it is to be hoped that some greatly improved system of gear change will, before long, supersede such a costly and barbarous system as the sliding gear. It is probable that a mechanical solution will be found in the application of the epicyclic gear, of carefully studied design, or by hydraulic or petrolelectric transmission. The increasing use of the multipledisc clutch, which has been rendered practicable by the invention of Dr. H. S. Hele-Shaw, F.R.S., is one of the notable features in vehicles of recent design.
Petrol-Electric Systems.
The electric vehicle, with the motors supplied from a battery of accumulators, has had practically no success in this country, although the advantages of electric, as compared with mechanical, transmission have not been lost sight of by several English bus constructors. At least two makers have, by recent demonstrations, proved that it is quite practicable to combine the best points of the petrol vehicle with a drive equally as flexible as that provided in the steam-driven vehicle. In such a machine, there is no grinding of gearing, no starting with a jerk, and, greatest advantage of all, perhaps, gradual and rapid acceleration from rest to normal speed. It is interesting to note that the subject of " Electrical Transmission Gears on Motor Vehicles "is occupying the attention of the electrical section of the engineering conference which is being held at the Institution of Civil Engineers this week. Mr. A. A. Campbell Swinton introduced the subject at last Wednesday's sitting, and a summary of his opening remarks will be found on page 384.
There are certain desirable features which should be aimed at in all passenger vehicles of the bus or char-a-bancs types, which are independent of the form of drive adopted. These principally refer to the body construction, springing, control of the vehicle, steering gear and brakes.
Overall 'Length of the Vehicle.
A chassis which can only offer a matter of fio per cent, of its total length for the accommodation of the body cannot be considered an ideal passenger vehicle. On the other Isand, a chassis which has the engine and other vital parts 'stowed away in most inaccessible positions is equally lacking, from a practical point of view. The combination of accessibility with the minimum overall length is a very worthy object, but, unfortunately, is only partly realised by a few designers and constructors.
Low Platform.
The comfort and safety of the passengers should be the first considerations, in the production of all vehicles of the types under consideration, in order that ingress and egress may be easy and quick for all users. The floor of the body elsouid be as low as is consistent with proper road clearance,
and there should be the least possible number of steps, which should be wide and deep, and with a fairly small
amount of rise. At least one designer has taken this matter seriously, and the " All British " bus chassis, which was described in last week's issue of this journal, is a step in the right direction, as far as lowering the platform is concerned. We reproduce a line drawing of a 36-seated bus of this make, from which the economy of space and low platform will be appreciated.
Best Type of Body.
This question must always be decided by the work for which the vehicle is required, and the conditions tinder whichit will be required to run. For public-service work in large towns it is more than probable that the present type of double-deck bus will hold its own, whilst, for the work of railway companies using motor vehicles as feeders to, or links between, loop and main lines, the single-deck or the composite body—part char-a-bancs and part single-deck---withluggage accommodation on the roof or in a roomy boot," will undoubtedly be found the most suitable. The Great Western Railway Company is at present operating vehicles with a new type of body. To all appearances these are double-deck bodies, but there are no seats on the top deck, which is reserved fur luggage (see page 391). The deep facia boards are retained, and they convert the, upper part of the body into a huge luggage well, access to which is obtained by a staircase. Side doors give entrance to the passenger compartment or compartments. For sight-seeing and pleasure purposes only, the char-as bancs will take a lot of beating for open country work, and wherever such vehicles have been introduced they have met with immense success.
Springing.
Easy riding, for which the method of spring suspension adopted is largely responsible, is a point to which but scant attention has been paid. Many makers have, however, realised that the three-point spring suspension, using a transverse spring, was the cause of much ominous and unpleasant rolling. It is a noticeable fact that, in all upte-date, passenger-vehicle chassis, long, flat, and flexible springs have replaced those previously used. The selection of the spring as far as thickness and number of plates are concerned, is not always all that is to be desired : not a few designers treat the spring only from the weight-carrying point of view, and give little, if any, consideration to the periods of oscillation and deflection.
Silent Running.
From the operating engineers' point of view, this is one of the most aggravating necessities of the motorbus. It is, none the less, a dominating one, when looked at from the standpoint of the Chief Commissioner of Police in London. Much has been, and is being done to remedy this evil, but the various and conflicting standards (sic) of noise do not make the production of the ideally-silent motor an easy task, although the degree of noise has been reduced more than fifty per cent. in the past 12 months. Steam and petrol-electric buses should not, of course, be nearly such great offenders in this respect as are some petrol buses.
Control of the Vehicle.
It is comforting to notice that, from many of the latest designs of well-known vehicles, the formidable array of lubricators, control levers, and other fittings have disappeared. The almost general adoption of pressure-feed lubrication, and the extended use of the magneto, are largely responsible for this simplification of the control systems. Practically the only matters over which the driver now has control are : the throttle-valve ; the clutch; the. changespeed ; the steering gear; and the brakes. The workmanship on the two last-mentioned units should be above suspicion, and the design should be such that there is little ikelihood of their being damaged by road shocks or sudden application.